Yes you can either use a USB wall socket and charge it that way (I personally use my Samsung phone charger) or if you have your bios setup correctly sometimes its possible to charge USB devices directly from your PC when it is switched off, but that is only a feature of certain motherboards. Otherwise you do need to have the PC switched on.
can u use it during charging process?
Yes you can!
are u able to charge the headset when the pc is turned off?
Yes you can either use a USB wall socket and charge it that way (I personally use my Samsung phone charger) or if you have your bios setup correctly sometimes its possible to charge USB devices directly from your PC when it is switched off, but that is only a feature of certain motherboards. Otherwise you do need to have the PC switched on.